Effects of Milk vs Dark Chocolate Consumption on Visual Acuity and Contrast Sensitivity Within 2 Hours: A Randomized Clinical Trial

Abstract

Importance: Consumption of dark chocolate can improve blood flow, mood, and cognition in the short term, but little is known about the possible effects of dark chocolate on visual performance.

Objective: To compare the short-term effects of consumption of dark chocolate with those of milk chocolate on visual acuity and large- and small-letter contrast sensitivity.

Design: A randomized, single-masked crossover design was used to assess short-term visual performance after consumption of a dark or a milk chocolate bar. Thirty participants without pathologic eye disease each consumed dark and milk chocolate in separate sessions, and within-participant paired comparisons were used to assess outcomes. Testing was conducted at the Rosenberg School of Optometry from June 25 to August 15, 2017.

Main outcomes and measures: Visual acuity (in logMAR units) and large- and small-letter contrast sensitivity (in the log of the inverse of the minimum detectable contrast [logCS units]) were measured 1.75 hours after consumption of dark and milk chocolate bars.

Results: Among the 30 participants (9 men and 21 women; mean [SD] age, 26 [5] years), small-letter contrast sensitivity was significantly higher after consumption of dark chocolate (mean [SE], 1.45 [0.04] logCS) vs milk chocolate (mean [SE], 1.30 [0.05] logCS; mean improvement, 0.15 logCS [95% CI, 0.08-0.22 logCS]; P < .001). Large-letter contrast sensitivity was slightly higher after consumption of dark chocolate (mean [SE], 2.05 [0.02] logCS) vs milk chocolate (mean [SE], 2.00 [0.02] logCS; mean improvement, 0.05 logCS [95% CI, 0.00-0.10 logCS]; P = .07). Visual acuity improved slightly after consumption of dark chocolate (mean [SE], -0.22 [0.01] logMAR; visual acuity, approximately 20/12) and milk chocolate (mean [SE], -0.18 [0.01] logMAR; visual acuity, approximately 20/15; mean improvement, 0.04 logMAR [95% CI, 0.02-0.06 logMAR]; P = .05). Composite scores combining results from all tests showed significant improvement after consumption of dark compared with milk chocolate (mean improvement, 0.20 log U [95% CI, 0.10-0.30 log U]; P < .001).

Conclusions and relevance: Contrast sensitivity and visual acuity were significantly higher 2 hours after consumption of a dark chocolate bar compared with a milk chocolate bar, but the duration of these effects and their influence in real-world performance await further testing.

Trial registration: clinicaltrials.gov Identifier: NCT03326934.

Figure 1.. CONSORT Diagram

At least 72 hours elapsed before the crossover was implemented.

Figure 2.. Bland-Altman Plot Showing the Difference in Composite Visual Score

The score difference is calculated for dark chocolate minus milk chocolate consumption plotted against the mean value for 30 participants with normal vision. The mean composite score is calculated using scores for dark and milk chocolate consumption for each participant. The 95% CI based on paired t test analysis is shown as dotted horizontal lines above and below the 0 line representing no difference in performance. Higher scores represent greater overall improvement in spatial vision after ingestion of dark vs milk chocolate.